Kaolin pigment products

ABSTRACT

A pigment product for use in a coating composition to provide a gloss coating on paper, the pigment product comprising a processed particulate kaolin having a particle size distribution such that at least about 80% by weight of the particles have an equivalent spherical diameter less than about 2 p,m and in the range of from about 10% to about 20% by weight of the particles have an equivalent spherical diameter less than about 0.25 wm, the particles have a shape factor in the range of from about 20 to about 36 and the particles have a particle steepness in the range of about greater than about 35, more preferably in the range of about 35 to about 40, and wherein the kaolin is derived from a secondary kaolin source.

FIELD OF THE INVENTION

The present invention relates to pigment products and their productionand use in coating compositions. The present invention further relatesto methods for making the pigment products and improved coated papermade using the pigment products.

The invention is concerned with the preparation of improved inorganicpigments for paper coating compositions, and, in particular, pigmentsfor use in compositions for preparing gloss coated paper.

BACKGROUND OF THE INVENTION

Paper coating compositions are generally prepared by forming a fluidaqueous suspension of pigment material together with a hydrophilicadhesive and other optional ingredients. Lightweight coated, or LWC,paper is generally coated to a weight of from about 5 g.m⁻² to about 20g.m⁻² on each side, and the total grammage, or weight per unit area ofthe coated paper is generally in the range of from about 49 g.m⁻² toabout 70 g.m⁻². The coating may conveniently be applied by means of acoating machine including a short dwell time coating head, which is adevice in which a captive pond of coating composition under a slightlyelevated pressure is held in contact with a moving paper web for a timein the range of from 0.0004 second to 0.01 second, before excess coatingcomposition is removed by means of a trailing blade. However, othertypes of coating apparatus may also be used for preparing lightweightcoated paper. LWC paper is generally used for printing-magazines,catalogues and advertising or promotional material. The coated paper isrequired to meet certain standards of surface gloss and smoothness. Forexample, the paper may have a gloss value of at least about 32, ae.g. upto about 50, TAPPI units, and a Parker Print Surf value is generally inthe range of from about 0.5 μm to about 1.6 μm.

Ultra lightweight coated, or ULWC, paper is sometimes otherwise known aslight lightweight coated, or LLWC, paper and is used for catalogues andfor advertising and promotional material sent through the mail to reducemailing costs. The coating weight is generally in the range of fromabout 3 g.m⁻² to about 8 g.m⁻² per side. The total grammage is generallyin the range of from about 30 g.m⁻² to about 48 g.m⁻².

An important white inorganic pigment for use in preparing coatingcompositions for the manufacture of LWC and ULWC papers is kaolinobtained from kaolin clay. Large deposits of kaolin clay exist in Devonand Cornwall, England and in the States of Georgia and South Carolina,United States of America. Important deposits also occur in Brazil,Australia, and in several other countries. Kaolin clay consistspredominantly of the mineral kaolinite, together with small proportionsof various impurities. Kaolinite exists in the form of hydrousaluminosilicate crystals in the shape of thin hexagonal plates, butthese plates tend to adhere together face-to-face to form stacks orbooklets. The individual plates may have mean diameters of 1 μm or less,but kaolinite particles in the form of booklets or stacks of plates mayhave an equivalent spherical diameter (esd) of up to 10 μm or more.Generally speaking, kaolin clay particles which have an equivalentspherical diameter of 2 μm or more are in the form of stacks ofkaolinite plates, rather than individual plates. Additionally, in somesecondary kaolin deposits smaller kaolin particles may sometimes adheretogether in random orientations to form agglomerated particles havingequivalent spherical diameters significantly greater than 2 μm.

The kaolin deposits in England differ from those in the United States ofAmerica and Brazil in that the English deposits are of primary kaolin,while those in the USA and Brazil are of the sedimentary or secondarytype. Kaolin was formed in geological times by the hydrothermaldecomposition of the feldspar component of granite, and primary kaolinis that which is obtained directly from the granite matrix in which itwas originally formed. On the other hand, secondary or (tertiary) kaolinalso known as sedimentary kaolin has been washed out of the originalgranite matrix in geological times and has been deposited in an arearemote from the site in which it was originally formed. Secondary kaolindeposits tend to have a higher proportion of fine particles, i.e., thosehaving an esd smaller than about 2 μm, because the kaolin has undergonea certain amount of natural grinding during the course of its transportfrom its site of origin to its site of final deposition. See, forexample, Jepson (Jepson, W. B., “Kaolins: their properties and uses”,Phil Trans R Soc Lond, A311, 1984, pp 411-432). Secondary kaolins alsotend to be more blocky than primary kaolins, containing large numbers ofboth kaolin stacks and agglomerates of smaller blocky kaolin particles.

As long ago as 1939, Maloney disclosed in U.S. Pat. No. 2,158,987 thatthe finish, or gloss, of a clay coated paper is greatly improved if theclay, before incorporation in the coating composition, is treated sothat a large percentage, for example 80% by weight or more, of the clayparticles have a size in the range of 0.1 μm to 2 μm. In order toincrease the proportion of fine particles in the raw kaolin, the rawkaolin may, according to the disclosure in U.S. Pat. No. 2,158,987, besubjected, before a centrifuging step, to a grinding operation in whicha suspension containing from about 50% to about 75% by dry weight ofkaolin and a dispersing agent is subjected to pebble milling. When thekaolin from the finer fraction is recovered, mixed with a suitable papercoating binder, and applied to the surface of a base paper, a coating ofgood gloss and color is obtained.

Various pigment products which are made using the principles describedby Maloney in U.S. Pat. No. 2,158,987 are commercially available andprovide good gloss and smoothness in coated papers, especially for LWCand ULWC paper. For example, a known pigment product available fromImerys, Inc., and recommended for gloss coatings of LWC comprises arefined English kaolin product having a particle size distribution,“psd”, such that 89% by weight of the particles have an esd less than 2μm, 74% by weight of the particles have an esd less than 1 μm and 25% byweight of the particles have an esd less than 0.25 μm.

One particularly useful kaolin pigment product which is commerciallyavailable fromlmerys, Inc. for use in gloss papers is marketed under thetradename of ‘SUPRAPRINT™’. SUPRAPRINT™ is derived from a coarse,primary English clay and generally has a particle size distribution suchthat about 90% by weight of the kaolin particles therein have an esdless that about 2 microns and about 12-16% by weight of the particleshave an esd less than about 0.25 microns. Further definingcharacteristics of the SUPRAPRINT™ kaolin include its shape factor inthe range of 30-35, and its particle steepness of approximately 40. Dueto its high steepness and shape factor and resultant particle packingeffects, the opacity and gloss of SUPRAPRINT™ have proven particularlysuited for use in gloss paper coatings and the like.

SUPRAPRINT™ is produced by from a Cornish primary clay having naturallyplaty fines-via the steps of: (1) pressing and pugging; (2) makedowninto a slurry; (3) mild grinding to partially delaminate a portion ofthe kaolin stacks present; (4) fractionation using a decantercentrifuge; and a series of beneficiation and finishing steps. A portionof the coarse fraction from step 4 is sometimes recycled to grindingstep 3 in an effort to maximize the yield of the process, but is notrequired to obtain product having the desired parameters. Though overallcoarse in nature, the primary Cornish clay used has a high concentrationof ultrafine platy particles which undesirably decrease the yield andincrease the amount of waste material that must be disposed of from theproduction process. Accordingly, production of the SUPRAPRINT™ productfrom coarse primary deposit of English kaolin has proven to beundesirably costly.

It would clearly be desirable to provide for the production of a pigmentproduct having similar parameters from a cheaper and easier to processsecondary kaolin deposit that has a much lower quantity of ultrafineplaty particles, such as can be found in the Rio Capim region of Brazil.Due to the inherently blocky nature of such secondary kaolins therequired processing would likely need to differ significantly from thatused to produce such a product from a more platy and coarse primarykaolin. Specifically, one might increase the shape factor of a blockysecondary kaolin through a harsher delamination that is typically usedfor the naturally platy primary kaolins. However, due to the presence ofagglomerates of blocky kaolin particles in such secondary kaolins, oneof ordinary skill in the art would expect typical delamination methodsto also result in an undesirable decrease in shape factor as theagglomerates break up into fine blocky particles instead ofdelaminating.

We have now developed an improved kaolin pigment product and method ofproduction thereof that addresses these shortcomings of the prior art.The present invention provides a pigment product having the desiredgloss and/or brightness, particularly, but which can be easily andinexpensively produced from a secondary kaolin deposit. In addition, thepigment product of the present invention may result in a reduction ofthe conventional amount of alternate pigments, for example, titaniumdioxide, that would be have to be used.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided apigment product for use in a coating composition to provide a glosscoating on paper, the pigment product comprising a processed particulatekaolin having a particle size distribution such that at least about 85%by weight of the particles have an equivalent spherical diameter lessthan 2 μm, and from about 15% to about 20% by weight of the particleshave an equivalent spherical diameter less than 0.25 μm, the particleshave a shape factor in the range of from about 20 to about 40, and theparticles have a particle steepness (d30/d70×100) in the range of about32 to about 40, and wherein said pigment product is derived from a crudekaolin obtained from a secondary kaolin deposit, such as for example thesecondary kaolin deposits found in the Rio Capim region of Brazil.

Surprisingly and beneficially, a pigment product according to the firstaspect of the invention can be produced easily and inexpensively from afine secondary kaolin while still providing improved gloss, brightnessand/or opacity when incorporated in a coating composition withoutundesirably increasing the presence of fine kaolin particles.Alternatively, the pigment product according to the first aspect of theinvention can retain the gloss, brightness, and/or opacitycharacteristics associated with prior products while naturally reducingthe amounts of expensive alternate pigments, e.g., titanium dioxide,that must be used, i.e., the same grade of coated paper may be producedat reduced cost.

The pigment product according to the first aspect of the invention hasan unexpected combination of properties which are not normally obtainedby conventional processing of secondary kaolin clays.

The following detailed description illustrates exemplary embodiments ofseveral aspects of the invention and serves merely to explain theprinciples of the invention rather than to cover every possibleembodiment thereof.

DESCRIPTION OF THE INVENTION

The pigment product according to the first aspect of the presentinvention may be produced by treating and blending particulate hydrouskaolin minerals of the secondary (aka sedimentary) type, moreparticularly raw or (partially) processed kaolin clays of a type thatcan be obtained from certain deposits in the Para State of Brazil suchas, for example, the Rio Capim watershed.

The pigment product according to the first aspect of the invention mayhave a particle size distribution, ‘psd’, such that in the range of fromabout 85% to about 95% by weight of the particles have an equivalentspherical diameter, ‘esd’, less than about 2 μm. In another embodiment,the range of from about 87% to about 93% by weight of the particles havean esd less than about 2 μm. In yet another embodiment, the range offrom about 89% to about 91% by weight of the particles have an esd lessthan about 2 μm. According to one embodiment of the present invention,the range of from about 15% to about 20% by weight of the particles havean esd less than about 0.25 μm. In yet another embodiment, the range offrom about 16% to about 18% by weight of the particles have an esd lessthan about 0.25 μm.

As will be appreciated by those skilled in the art, the psd of aparticulate product such as the pigment product according to the presentinvention may be determined by measuring the sedimentation speeds of thedispersed particles of the particulate product under test through astandard dilute aqueous suspension using a SEDIGRAPH™ machine, e.g.,SEDIGRAPH 5100, obtained from Micromeritics Corporation, USA. The sizeof a given particle is expressed in terms of the diameter of a sphere ofequivalent diameter, which sediments through the suspension, i.e., anequivalent spherical diameter or esd, the parameter as referred toabove. The SEDIGRAPH machine graphically records the percentage byweight of particles having esd less than a certain esd value versus esd.

A kaolin product of high shape factor is considered to be more “platy”than a kaolin product of low shape factor. “Shape factor” as used hereinis a measure of an average value (on a weight average basis) of theratio of mean particle diameter to particle thickness for a populationof particles of varying size and shape as measured using the electricalconductivity method and apparatus described in GB-A-2240398/U.S. Pat.No. 5,128,606/EP-A-0528078, which are incorporated herein by referencein their entirety, and using the equations derived in these patentspecifications. “Mean particle diameter” is defined as the diameter of acircle which has the same area as the largest face of the particle. Inthe measurement method described in EP-A-0528078 the electricalconductivity of a fully dispersed aqueous suspension of the particlesunder test is caused to flow through an elongated tube. Measurements ofthe electrical conductivity are taken between (a) a pair of electrodesseparated from one another along the longitudinal axis of the tube, and(b) a pair of electrodes separated from one another across thetransverse width of the tube, and using the difference between the twoconductivity measurements, the shape factor of the particulate materialunder test is determined.

According to the first aspect of the invention, the pigment product ofthe present invention comprises particles having a shape factor in therange of from about 20 to about 40, for example from about 20 to about36. In another embodiment according to the present invention, the shapefactor may be in the range of from about 23 to about 33. In stillanother embodiment, the shape factor may be in the range of from about25 to about 29. In another embodiment, the shape factor is in the rangeof from about 26 to 28.

Additionally, in accordance with the above described embodiments of thefirst aspect of the present invention, the pigment particles may have aparticle steepness (d30/d70×100, where d30 is the particle diametermeasured by the SediGraph instrument at 30% weight passing below thediameter and where d70 is the particle diameter measured at 70% weightpassing below the diameter) of greater than 32, for example greater than35. Alternatively, the particles may have a more narrow particlesteepness of between 32 and 40, between 35 and 40, between 36 and 39, oreven of about 38.

According to the present invention in a second-aspect there is provideda method for producing a pigment product according to the first aspect,the method comprising the steps of:

-   (a) mixing a raw or partially processed kaolin clay with water to    form an aqueous suspension;-   (b) blending said aqueous suspension with a coarse kaolin to form a    blended kaolin suspension comprising kaolin particles that include    no more than about 60% by weight particles having an esd less than 2    microns;-   (c) subjecting the blended kaolin suspension produced by step (b) to    attrition grinding using a particulate grinding medium by a process    in which the average shape factor of the kaolin is increased by, for    example, at least about 10 to at least about 15;-   (d) separating the suspension of ground kaolin from the particulate    grinding medium; and-   (e) fractionating the suspension of ground kaolin separated in    step (d) to recover a kaolin pigment having a desired particle size    distribution therefrom.

In step (a) of the method according to the second aspect of the presentinvention, the kaolin clay may form from 20% to 70%, usually from 20% to45% of the treated suspension. The kaolin clay preferably comprises asecondary (aka sedimentary) kaolin clay, such as a secondary kaolin clayfrom the Para State region of Brazil, e.g. from the Rio Capim watershedregion. The raw kaolin clay may be a fine kaolin having a psd such thatnot more than about 40% by weight comprising particles having an esdlarger than 1 Opn and not more than 75% by weight, e.g. from about 50%to about 70% by weight, consists of particles having an esd smaller than2 μm. The shape factor of the kaolin clay treated in step (a) may beless than 15, e.g. in the range of from about 5 to about 12.

In step (b) the psd of the kaolin clay may be adjusted so that it is inaccordance with the first aspect of the invention by blending from 90 to20 parts by weight of degritted (substantially all particles greaterthan 45 microns are removed by hydrocyclones, dragboxes and/orscreens)—unfractionated (not centrifuged except for the purpose ofscalping to remove substantially all particles greater than 45 micron)kaolin clay with from 10 to 80 parts by weight, for example from 40 to60 parts by weight, of at least one fractionated coarse kaolincomponent, for example a coarse fraction from a centrifugalclassification step of a related or unrelated kaolin product. The coarsefraction may be derived from a kaolin which has been previouslysubjected to attrition grinding, magnetic separation and sizeclassification steps. An exemplary coarse fraction may, for example havea shape factor ranging from about 5 to about 55, for example from about10 to about 30, or even from about 10 to about 12, and a percentage byweight of particles smaller than 2 μm ranging from about 20% to about40%, for example from about 25 to about 35%, or even from about 30% toabout 35%. The addition may be carried out with the kaolin to be blendedin either powdered, dry form or in the form of an aqueous suspension. Inone exemplary embodiment, the coarse fractions are routed directly fromone or more centrifugal classifiers to a blend tank for mixture with acrude kaolin suspension that has been previously subjected todegritting, a low energy scrub grind in an stirred media mill orattrition grinder, and magnetic separation.

We have found that by producing a kaolin product by blending in themanner described, the rheology and dewatering characteristics of anaqueous suspension of the kaolin processed in accordance with the methodof the second aspect are improved, giving better runnability andparticle alignment when the kaolin is used in a coating composition asdescribed later. Thus, it may in some instances be advantageous to moreprecisely control the characteristics of the blended kaolin by addingmore precisely metered amounts of the one or more coarse kaolin beingadded to the blend tank. This can easily be accomplished by divertingthe centrifuge coarse fractions into intermediate storage tanks fromwhich the rate of addition may be more easily metered.

While not intending to be bound by any one theory, it is our hypothesisthat the coarse fractions used in the exemplary embodiments of thepresent invention are particularly suited for later delamination becausethey are enriched in coarse kaolin particles comprising kaolin booklets.At least in the exemplary embodiments, the coarse kaolin may be derivedby the size classification of pre-ground kaolin. We hypothesise that adisproportionate number of the coarse particles broken up by the initialgrinding comprise aggregates of smaller blocky kaolin particles and thatthe kaolin booklets present are largely not delaminated in the initialgrinding step. Thus, by first grinding and size classifying to obtainthe coarse kaolin used in the exemplary embodiments of the presentinvention, it may be possible to effectively enrich the number of kaolinbooklets suitable for delamination, thereby increasing the shape factorof the resultant product.

The kaolin clay treated in the method according to the second aspect ofthe invention may be subjected to one or more well known purificationsteps to remove undesirable impurities, e.g. between steps (a) and (c).For example, the aqueous suspension of kaolin clay may be subjected to afroth flotation treatment operation to remove titanium containingimpurities in the froth. Alternatively, or in addition, the suspensionmay be passed through a high intensity magnetic separator to remove ironcontaining impurities.

Step (c) may comprise a process wherein the suspension of kaolin clay istreated by medium attrition grinding wherein an energy of from about 40kWh to about 250 kWh per tonne of clay (on a dry weight basis) isdissipated in the suspension. The attrition grinding can be carried outin one grinding stage or in multiple grinding stages. In one embodimentof the present invention, the attrition grinding has been achieved usingan attrition scrubber, such as the Floatex Attrition Scrubber availablefrom the Carpco Division of Outokumpu Technology, Inc. This attritiongrinding step has the general result of increasing the average shapefactor of the kaolin due to delamination of the kaolin books and stackspresent in the suspension. The particulate grinding medium is suitablyof relatively high specific gravity, for example 2 or more, and may forexample comprise grains of silica sand, the grains generally havingdiameters not larger than about 2mm and not smaller than about 0.25 mm.Suitably, attrition grinding step (c) will be sufficient to increase theshape factor of the kaolin clay by at least about 10, for example by atleast about 15.

In step (d) of the method according to the second aspect of theinvention, the suspension of ground kaolin clay may be convenientlyseparated from the particulate grinding medium in a known manner, e.g.by passing the suspension through a sieve of appropriate aperture size,for example a sieve having nominal aperture sizes in the range of fromabout 0.1 mm to about 0.25 mm.

In step (e) of the method according to the second aspect of theinvention, the suspension of ground kaolin clay may be fractionatedbased upon particle size into coarse and fine fractions. Fractionation(or classification) may be accomplished using any known orafter-discovered method. Appropriate methods include gravitysedimentation or elutriation, any type of hydrocyclone apparatus, or,suitably, a solid bowl decanter centrifuge, disc nozzle centrifuge, orthe like. The resultant coarse fraction may be discarded, used as aseparate product or may be directed back to the blend tank as anauxiliary source for the coarse kaolin used in the method of the presentinvention. The coarse fraction may suitably have a psd such that no morethan about 60% by weight of the particles therein have an esd of lessthan 2 microns and may have a shape factor of at least about 40.

Following step (e) of the method according to the second aspect of theinvention, the resultant fine fraction of suspended kaolin may comprisethe kaolin composition of the first aspect of the invention. The finefraction of suspended kaolin may optionally be subjected to aconventional leaching (or bleaching) step that should be familiar tothose of ordinary skill in the art. Specifically, a reductive leachingagent such as sodium hydrosulfite may be added in a dose range of, forexample, about 0.5 to 5.0 pounds per ton of kaolin, with certainembodiments containing less than about 4 pounds per ton on a dry basis.Other suitable reductive bleaching agents, such as formamidine sulphonicacid, may also be employed. Reductive bleaching using sodiumhydrosulfite is suitably carried out at an acidic pH,-typically in therange of 2.0 to 4.0.

The fine fraction of suspended ground kaolin may also optionally bedewatered in one of the ways well known in the art, e.g. filtration,centrifugation, evaporation and the like. In one aspect, a filter pressis used to dewater the suspended ground kaolin to produce a filter cake.The filter cake may be mixed with a dispersing agent for the kaolin clayand thus converted into a fluid slurry which may be transported and soldin this form. Alternatively, the kaolin clay may be thermally dried, forexample by introducing the fluid slurry of the kaolin clay into a spraydrier and thereby transported in a substantially dry form.

Following step (d) or step (e) the kaolin clay may be further treated toimprove one or more of its properties. For example high energy liquidworking, e.g. using a high speed mixer, may be applied to the product inslurry form, e.g. before step (e) or after step (e) and subsequentre-dispersion in an aqueous medium, e.g. during makedown of a coatingcomposition.

Use of the pigment product provided by the invention in a paper coatingcomposition may advantageously result in a reduction of the conventionalamount of titanium dioxide pigment and plastic pigment (hollow core andsolid core) which would typically be used in a paper coatingcomposition, such as up to about 4 parts by weight of each pigment basedon the weight of the pigment product. Titanium dioxide and plasticpigment are relatively expensive pigments and the latter pigment maycause undesirable viscosity problems in coating compositions.

The pigment product according to the first aspect of the presentinvention may be used in paper coating as follows.

According to the present invention in a third aspect there is provided acoating composition for use in producing gloss coatings on paper andother substrates which composition comprises an aqueous suspension of aparticulate pigment and a hydrophilic adhesive or binder, wherein theparticulate pigment comprises the pigment in accordance with the firstaspect of the invention. In this specification, the expression ‘paper’embraces products which are of paper, board, card and the like.

The solids content of the paper coating composition according to thethird aspect of the invention may be greater than about 60% by weight,e.g. at least 70%. The composition may include a dispersing agent, e.g.,up to about 2% by weight of a polyelectrolyte based on the dry weight ofpigment present. For example, polyacrylates and copolymers containingpolyacrylate units are well known as suitable polyelectrolytes. Thepigment product according to the first aspect of the invention may beused as the sole pigment in the paper coating composition according tothe third aspect, or it may be used in conjunction with one or moreother known pigments, such as for example, (commercially available)kaolin, calcined kaolin, natural or precipitated calcium carbonate,titanium dioxide, calcium sulphate, satin white, talc and so called‘plastic pigment’. When a mixture of pigments is used the pigmentproduct according to the first aspect of the invention is suitablypresent in the mixture in an amount of at least about 80% of the totaldry weight of the mixed pigments.

The binder of the composition according to the third aspect may comprisean adhesive derived from natural starch obtained from a known plantsource, for example, wheat, maize, potato or tapioca, although it is notessential to use starch as a binder ingredient. Other binders, which maybe used with or without starch, are mentioned later.

Where starch is employed as a binder ingredient, the starch may beunmodified or raw starch, or it may be modified by one or more chemicaltreatments known in the art. The starch may, for example, be oxidized toconvert some of its —CH₂OH groups to —COOH groups. In some cases thestarch may have a small proportion of acetyl-COCH₃, groups.Alternatively, the starch may be chemically treated to render itcationic or amphoteric, i.e., with both cationic and anionic charges.The starch may also be converted to a starch ether, or hydroxyalkylatedstarch by replacing some —OH groups with, for example, —O—CH₂—CH₂OHgroups, —O—CH₂—CH₃ groups or —O—CH₂—CH₂—CH₂OH groups. A further class ofchemically treated starches which may be used are those known as thestarch phosphates. Alternatively, the raw starch may be hydrolyzed bymeans of a dilute acid or an enzyme to produce a gum of the dextrintype. The amount of the starch binder used in the composition accordingto the third aspect may range from, for example, about 2% to about 25%by weight, based on the dry weight of pigment. The starch binder may beused in conjunction with one or more other binders, for examplesynthetic binders of the latex or polyvinyl acetate or polyvinyl alcoholtype. When the starch binder is used in conjunction with another binder,e.g., a synthetic binder, the amount of the starch binder may rangefrom, for example, about 2% to about 20% by weight, and the amount ofthe synthetic binder from about 2% to about 12% by weight, both based onthe weight of dry pigment. In one embodiment, at least about 50% byweight of the binder mixture comprises modified or unmodified starch.

According to the present invention in a fourth aspect there is provideda method of use of the coating composition according to the third aspectwhich comprises applying the composition to coat a sheet of paper andcalendering the paper to form a gloss coating thereon. In someembodiments, the gloss coating is formed on both sides of the paper.

Calendering is a well known process in which paper smoothness and glossis improved and bulk is reduced by passing a coated paper sheet betweencalender nips or rollers one or more times. Usually, elastomer coatedrolls are employed to give pressing of high solids compositions. Anelevated temperature may be applied. Five or more passes through thenips may be applied.

The paper after coating and calendering in the method according to thefourth aspect may have a total weight per unit area in the range about30 g.m⁻² to about 70 g.m^(−2.) In another embodiment the total weightper unit is in the range about 49 g.m⁻² to about 65 g.m⁻² or about 35g.m⁻² to about 48 g.m⁻². The final coating may have a weight per unitarea of from about 3 g.m⁻² to about 20 g.m⁻². In yet another embodimentthe final coating weight is from about 5 g.m⁻² to about 13 g.m⁻² for LWCand about 4 g.m⁻² to about 8 g.m⁻² for ULWC. Such a coating may beapplied to both sides of the paper. Thus, the coated paper may be LWC orULWC paper. The paper gloss may be greater than about 45 TAPPI units andthe Parker Print Surf value at a pressure of 1 MPa of each paper coatingmay be less than about 1 μm.

The gloss of a coated paper surface may be measured by means of a testlaid down in TAPPI Standard No 480 ts-65. The intensity of lightreflected at an angle from the surface of the paper is measured andcompared with a standard of known gloss value. The beams of incident andreflected light are both at an angle of 75° to the normal to the papersurface. The results are expressed in TAPPI gloss units. The gloss ofthe pigment product according to the first aspect may be greater thanabout 50, in some cases greater than about 55, TAPPI units.

The Parker Print Surf test provides a measure of the smoothness of apaper surface, and comprises measuring the rate at which air underpressure leaks from a sample of the coated paper which is clamped, undera known standard force, between an upper plate which incorporates anoutlet for the compressed air and a lower plate, the upper surface ofwhich is covered with a sheet of either a soft or a hard referencesupporting material according to the nature of the paper under test.From the rate of escape of the air, a root mean cube gap in μm betweenthe paper surface and the reference material is calculated. A smallervalue of this gap represents a higher degree of smoothness of thesurface of the paper under test.

An improvement is provided by the present invention where the binderpresent in the coating composition according to the third aspectcomprises starch. However, an improvement is also obtained where otherknown starch-free binders are employed (with or without starch present).In each case the adhesive or binder may form from about 4% to about 30%by weight of the solids content of the composition. In anotherembodiment, the adhesive or binder may be from about 8% to about 20% byweight of the solids content of the composition. In yet anotherembodiment, the adhesive or binder may be from about 8% to about 15% byweight of the solids content of the composition. The amount employedwill depend upon the composition and the type of adhesive, which mayitself incorporate one or more ingredients. For example, hydrophilicadhesives used in the art, e.g., incorporating one or more of thefollowing adhesive or binder ingredients may be used in the followingstated amounts:

(a) latex: levels range from about 4% by weight to about 20% by weight.The latex may comprise, for example, a styrene butadiene, acrylic latex,vinyl acetate latex, or styrene acrylic copolymers.

(b) other binders: levels range from, for example about 4% by weight toabout 20% by weight. Examples of other binders include casein, polyvinylalcohol and polyvinyl acetate.

Additives in various known classes may, depending upon the type ofcoating and the material to be coated, be included in the coatingcomposition according to the third aspect of the present invention.Examples of such classes of optional additive are as follows:

(a) cross linkers: e.g., in levels of up to about 5% by weight; forexample glyoxals, melamine formaldehyde resins, ammonium zirconiumcarbonates.

(b) water retention aids: e.g., in up to about 2% by weight, forexample, sodium carboxymethyl cellulose, hydroxyethyl cellulose, PVA(polyvinyl acetate), starches, proteins, polyacrylates, gums, alginates,polyacrylamide bentonite and other commercially available products soldfor such applications.

(c) viscosity modifiers or thickeners: e.g., in levels up to about 2% byweight; for example, polyacrylates, emulsion copolymers, dicyanamide,triols, polyoxyethylene ether, urea, sulphated castor oil, polyvinylpyrrolidone, montmorillonite, CMC (carboxymethyl celluloses), sodiumalginate, xanthan gum, sodium silicate, acrylic acid copolymers, HMC(hydroxymethyl celluloses), HEC (hydroxyethyl celluloses) and others.

(d) lubricity/calendering aids: e.g., in levels up to about 2% byweight, for example, calcium stearate, ammonium stearate, zinc stearate,wax emulsions, waxes, alkyl ketene dimer, glycols.

(e) dispersants: e.g., in levels up to about 2% by weight, for example,polyelectrolytes such as polyacrylates and copolymers containingpolyacrylate species, more particularly, polyacrylate salts (e.g.,sodium and aluminium optionally with a group II metal salt), sodiumhexametaphosphates, non-ionic polyol, polyphosphoric acid, condensedsodium phosphate, non-ionic surfactants, alkanolamine and other reagentscommonly used for this function.

(f) antifoamers/defoamers: e.g., in levels up to about 1% by weight, forexample, blends of surfactants, tributyl phosphate, fattypolyoxyethylene esters plus fatty alcohols, fatty acid soaps, siliconeemulsions and other silicone containing compositions, waxes andinorganic particulates in mineral oil, blends of emulsified hydrocarbonsand other compounds sold commercially to carry out this function.

(g) dry or wet pick improvement additives: e.g., in levels up to about2% by weight, for example, melamine resin, polyethylene emulsions, ureaformaldehyde, melamine formaldehyde, polyamide, calcium stearate,styrene maleic anhydride and others.

(h) dry or wet rub improvement and abrasion resistance additives: e.g.,in levels up to about 2% by weight, for example, glyoxal based resins,oxidized polyethylenes, melamine resins, urea formaldehyde, melamineformaldehyde, polyethylene wax, calcium stearate and others.

(i) gloss-ink hold-out additives: e.g., in levels up to about 2% byweight, for example, oxidized polyethylenes, polyethylene emulsions,waxes, casein, guar gum, CMC, HMC, calcium stearate, ammonium stearate,sodium alginate and others.

(j) optical brightening agents (OBA) and fluorescent whitening agents(FWA): e.g., in levels up to about 1% by weight, for example stilbenederivatives.

(k) dyes: e.g., in levels up to about 0.5% by weight.

(l) biocides/spoilage control agents: e.g. in levels up to 1% by weight,for example, metaborate, sodium dodecylbenzene sulphonate, thiocyanate,organosulphur, sodium benzonate and other compounds sold commerciallyfor this function.

(m) leveling and evening aids: e.g., in levels up to about 2% by weight,for example, non-ionic polyol, polyethylene emulsions, fatty acid,esters and alcohol derivatives, alcohol/ethylene oxide, sodium CMC, HEC,alginates, calcium stearate and other compounds sold commercially forthis function.

(n) grease and oil resistance additives: e.g., in levels up to about 2%by weight, e.g., oxidized polyethylenes, latex, SMA (styrene maleicanhydride), polyamide, waxes, alginate, protein, CMC, HMC.

(o) water resistance additives: e.g., in levels up to about 2% byweight, e.g., oxidized polyethylenes, ketone resin, anionic latex,polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde, melamineformaldehyde, polyamide, glyoxals, stearates and other materialscommercially available for this function.

(p) insolubilizer: e.g., in levels up to about 2% by weight.

For all of the above additives, the percentages by weight quoted arebased on the dry weight of pigment (100%) present in the composition.Where the additive is present in a minimum amount the minimum amount maybe 0.01% by weight based on the dry weight of pigment.

The method according to the fourth aspect of the present invention maybe carried out in a known way which will depend upon the material to becoated, the coating composition to be applied and other factors asdetermined by the operator, e.g., speed and ease of runnability e.g.,using a conventional coating machine.

Methods of coating paper and other sheet materials are widely publishedand well known. For example, there is a review of such methods publishedin Pulp and Paper International, May 1994, page 18 et seq. Sheets may becoated on the sheet forming machine, i.e., “on-machine”, or“off-machine” on a coater or coating machine. Use of high solidscompositions is desirable in the coating method because it leaves lesswater to evaporate subsequently. However, as is well known in the art,the solids level should not be so high that high viscosity and levelingproblems are introduced.

According to the fourth aspect of the present invention a paper coatingmethod may include: (i) a means of applying the coating composition tothe material to be coated, viz., an applicator; and (ii) a means forensuring that a correct level of coating composition is applied, viz., ametering device. When an excess of coating composition is applied to theapplicator, the metering device is downstream of it. Alternatively, thecorrect amount of coating composition may be applied to the applicatorby the metering device, e.g., as a film press. At the points of coatingapplication and metering, the paper web support ranges from a backingroll, e.g., via one or two applicators, to nothing i.e., just tension.The time the coating is in contact with the paper before the excess isfinally removed is the dwell time—and this may be short, long orvariable.

The coating is usually added by a coating head at a coating station.According to the quality desired, paper grades are uncoated, singlecoated, double coated and even triple coated. When providing more thanone coat, the initial coat (precoat) may have a cheaper formulation andoptionally less pigment in the coating composition. A coater that isapplying a double coating, i.e., a coating on each side of the paper,will have two or four coating heads, depending on the number of sidescoated by each head. Most coating heads coat only one side at a time,but some roll coaters (e.g., film press, gate roll, size press) coatboth sides in one pass.

Examples of known coaters which may be employed include air knifecoaters, blade coaters, rod coaters, bar coaters, multi-head coaters,roll coaters, roll/blade coaters, cast coaters, laboratory coaters,gravure coaters, kiss coaters, liquid application systems, reverse rollcoaters and extrusion coaters.

In all examples of coating compositions described in this specification,water is added to the solids to give a concentration of solids whichwhen coated onto a sheet to a desired target coat weight has a rheologysuitable for the composition to be coated with a pressure (e.g., a bladepressure) of between about 1 and about 1.5 bar. Generally, the solidscontent may be from about 60% to about 70% by weight.

Embodiments of the present invention will now be described by way ofexample only with reference to the following illustrative Examples.

EXAMPLE 1 Preparation

A raw kaolin clay from a sedimentary deposit in the Rio Capim region ofBrazil was suspended in water, blunged at 40% to 45% solids, degrittedand scrub ground according to conventional methods that would befamiliar to one of ordinary skill in the art. The resultant suspensionwas passed through a high intensity magnetic separator to removeiron-containing impurities therefrom. After the magnetic separation stepthe kaolin clay was found to have a psd such that 68% by weightconsisted of particles having an esd smaller than 2 μm. The shape factorof the kaolin clay as measured by the method described in GB-A-2240398was found to be about 11.

A first portion of the magnetted crude kaolin suspension was directed toa Floatex Attrition Scrubber (available from the Carpco Division ofOutokumpu Technology, Inc.) and ground for a time such thatapproximately 50 kWh of energy was dissipated per tonne of kaolin clay.The grinding medium was a silica sand having grains in the size rangefrom about 0.6 mm to about 0.85 mm. At the completion of this grindingstep, the kaolin clay was found to have a psd such that 70-75% by weightconsisted of particles having an esd smaller than 2 μm. The meanparticle shape factor as measured by the method described in GBA-2240398was found to be 38-42.

The sandgrinder product had a psd such that approximately 70-75% byweight of the particles therein had an esd less than about 2 microns,and a shape factor in the range of 38-40.

The suspension of ground kaolin clay was separated from the grindingmedium by elutriation to recover a delaminated kaolin stream. Thisdelaminated kaolin stream was then directed to a Bird-type centrifugeand separated into coarse and fine fractions. The resultant finefraction was used as an end product unrelated to the kaolins of thepresent invention. The resultant coarse fraction consisted of a kaolinsuspension having approximately 34% of particles by weight less than 2microns and a shape factor of approximately 12. It also is hypothesizedthat this coarse fraction was enriched in kaolin booklets and stacks aspreviously discussed. The coarse fraction was directed to the blend tankfor mixing with a second portion of the magnetted crude in a ratio ofabout 20% magnetted crude to about 80% coarse kaolin suspension to forma blended kaolin suspension.

The overflow product from the centrifuge was then acidified withsulphuric acid, bleached with 3 kg of sodium dithionite bleachingreagent per tonne of dry kaolin, dewatered in a filterpress, and thecake thus formed was redispersed with the aid of 0.25% by weight, basedon the weight of dry kaolin, of a sodium polyacrylate dispersing agent.The dispersed suspension was then spray dried to form a dry powder.

The final kaolin clay product had a psd such that 89-92% by weightconsisted of particles having an esd smaller than 2 μm, and about 15-19%by weight consisted of particles having an esd smaller than 0.25 μm. Theshape factor of the product as measured by the method described inGB-A-2240398 was found to be greater than 26 (up to about 35, dependingon the particular batch tested). The particle steepness of the productwas approximately in the range of about 35 to about 40.

This product was identified as kaolin compositions A1 through A3 whichrespectively reflect different batches of product produced by the abovedetailed method.

EXAMPLE 2 Kaolin Properties

The brightness (ISO), yellowness, particle size distribution, shapefactor and viscosity concentrations of kaolin compositions A1-A3 arecompared in Table 1 to those of the prior art composition SUPRAPRINT™.

TABLE 1 Kaolin Properties Viscosity Brightness <2 μm <1 μm <0.5 μm <0.25μm Shape Concentration Pigment (ISO) Y'ness (%) (%) (%) (%) Factor (%)SUPRAPRINT ™ 88.4 3.6 92 71 38 15 35 70 Composition A3 89.1 4.4 92 72 4519 26 72.3 (Invention) Composition A3 88.6 4.7 89 66 39 17 31 71.9(Invention) Composition A3 89.4 4.3 90 67 40 16 32 71.7 (Invention)

As can be seen in Table 1, the secondary kaolins prepared in accordancewith aspects of the present invention have properties that are overallclose to the desired properties of the prior art primary kaolin. Whilethe inventive kaolins do have a somewhat lower shape factor and aslightly more yellow tint that the prior art primary kaolin, they have acomparable psd and superior brightness and viscosity concentrations.Brightness is defined as the percentage reflectance to light ofwavelength 457 nm as measured in accordance with International StandardNo ISO 2470 and powder yellowness is defined as the difference betweenthe percentage reflectance of the powder to light of wavelength 457 nmand the percentage reflectance to light of wavelength 570 nm, bothmeasured in accordance with International Standard No ISO 2470.Viscosity concentration was determined as the solids concentrationnecessary to generate a Brookfield viscosity reading of five (5) poise.

EXAMPLE 3 Coating Colour Properties

The pigments were made down and coated onto a 36 g/m² mechanical base(reel no. 500) at 800 m/min using the short dwell time applicator onHelicoater 2000C, in a typical 100% kaolin LWC offset formulation, asshown in Table 2.

TABLE 2 Coating colour formulations Pigment 1 2 3 4 SUPRAPRINT ™ 100Composition A1 100 Composition A2 100 Composition A3 100 Dow 920 Latex11 11 11 11 CMC FF10 0.3 0.3 0.3 0.3

Coatings were applied at the highest runnable solids, at which colourproperties are shown in Table 3. Coatweights in the range 5 to 12 g/m²were applied.

TABLE 3 Coating colour properties Eklund Brookfield Viscosity BohlinWater Solids (mPa · s) mPa · s @ loss Colour (%) 10 rpm 20 rpm 50 rpm100 rpm 13000 sec−¹ g/m² SUPRAPRINT ™ 62.2 2800 1600 800 480 180 88Coating Colour Coating Colour A1 64.2 3200 1800 880 540 111 108 CoatingColour A2 63.7 3200 1800 880 540 131 97 Coating Colour A3 63.7 3400 1900920 560 176 97

Increasing the shape factor of the two recent batches of the coatingcolours made with the secondary kaolins of the present invention reducedthe runnable solids of the coating colours by about 0.5%, but the solidslevels was still significantly higher (1.5%) than that of the prior artprimary kaolin. The Brookfield viscosity measurements of the coatingcolour containing secondary kaolins prepared in accordance with thepresent invention were similar to that of the coating colours containingprior art primary kaolin. The Bohlin number was determined using a highshear viscometer in accordance with conventional methods that should beknown to one of ordinary skill in the art. Generally speaking, a lowerBohlin number is preferable for a coating colour. Thus, the inventivecompositions show superior performance over the prior art primary kaolincomposition when used in coating colours. Water retention properties ofthe coating colours prepared using the kaolins of the present inventionwere slightly less favourable than those prepared using the prior artprimary kaolin. Thus, overall the measured parameters are quite close tothe desired characteristics of the coating colours prepared using theprior art primary derived kaolin.

EXAMPLE 4 Coated Paper Properties

To further illustrate the utility of the kaolin compositions prepared inaccordance with aspects of the present invention, coated paper testswere also conducted. Values interpolated to a coating thickness of 8g/m² are given in Table 4.

TABLE 4 Paper properties interpolated to 8 g/m² Pigment Paper ShapeGurley Pigment B'ness Factor B'ness Opacity PPS 10 kg Gloss Porosity (s)SUPRAPRINT ™ 88.4 35 71.2 87.3 0.84 75 4000 Composition A1 89.1 26 71.687.3 0.82 79 4000 Composition A2 88.6 31 71.3 87.2 0.83 80 5000Composition A3 89.4 32 71.6 87.0 0.83 80 5000

As can be seen in Table 4, the brightness, opacity and Parker Print Surfvalues of papers prepared using secondary kaolin compositions preparedin accordance to aspects of the present invention were all comparable tothose prepared using the prior art primary kaolin composition. Theporosity paper made using the inventive compositions ranged fromidentical to, to slightly inferior to, that of paper made using theprior art composition. The brightness and gloss achieved with theinventive compositions were superior to those achieved using the priorart primary kaolin composition.

EXAMPLE 5 Offset Printing Properties

As can be seen in Table 5, coated papers produced using the secondarykaolin derived compositions of the present invention perform remarkablysimilarly to the prior art primary derived kaolins with respect to theiroffset printing properties.

TABLE 5 Offset printing properties of ca. 8 g/m² coatweight papers DeltaGloss Print Gloss (Dry- Print Density Cyan Pigment Dry Litho Paper) DryLitho L/D Mottle SUPRAPRINT ™ 86 78 11 1.44 1.31 0.91 Slight CompositionA1 85 79 6 1.43 1.32 0.92 Slight Composition A2 86 78 6 1.45 1.32 0.91Slight Composition A3 86 78 6 1.46 1.32 0.90 Slight

Print gloss was measured in accordance with the gloss measurementprocedures previously discussed and the delta gloss represents thechange in gloss from before to after printing. The dry print density,again measured according to industry standard procedures, is determinedby measuring the reflectance of white light from the sample under testand from an unprinted white sheet and applying the following formula:Density (D)=Log₁₀ 1/R

-   -   where R═R₁÷R_(w)        and where R₁ is the intensity of light reflected by the sample        under test and R_(w) is the intensity of light reflected by the        unprinted white sheet.

Thus, it can be seen from the above examples, that a kaolin product canbe produced from a secondary kaolin in accordance with the methods ofthe present inventions that is a suitable substitute for the prior artkaolin products having desired properties that have been previously onlyderived from primary kaolins.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A pigment product comprising kaolin having a particle sizedistribution such that at least about 85% by weight of the particleshave an equivalent spherical diameter less than about 2 μm, and fromgreater than 10% to about 20% by weight of the particles have anequivalent spherical diameter less than 0.25 μm, the particles have ashape factor in the range of from about 26 to about 40, and theparticles have a particle steepness of greater than about 32, whereinsaid kaolin is derived from a crude secondary kaolin.
 2. The pigmentproduct according to claim 1, wherein said crude secondary kaolin isobtained from the Para State region of Brazil.
 3. The pigment productaccording to claim 1, wherein from about 85% to about 95% by weight ofthe particles have an equivalent spherical diameter less than about 2μm.
 4. The pigment product according to claim 1, wherein from about 87%to about 93% by weight of the particles have an equivalent sphericaldiameter less than about 2 μm.
 5. The pigment product according to claim1, wherein from about 14% to about 18% by weight of the particles havean equivalent spherical diameter less than about 0.25 μm.
 6. The pigmentproduct according to claim 1, wherein the shape factor of the particlesis in the range of from about 20 to about
 36. 7. The pigment productaccording to claim 1, wherein the shape factor of the particles is inthe range of from about 26 to about
 33. 8. The pigment product accordingto claim 7, wherein the shape factor of the particles is in the range offrom about 26 to about
 30. 9. The pigment product according to claim 8,wherein the shape factor of the particles is in the range of from about28 to about
 30. 10. The pigment product according to claim 1, whereinthe steepness of the particles is in the range of from about 32 to about40.
 11. The pigment product according to claim 1, wherein the steepnessof the particles is in the range of from about 35 to about
 39. 12. Thepigment product according to claim 1, wherein the steepness of theparticles is in the range from about 36 to about
 38. 13. The pigmentproduct according to claim 1, wherein said kaolin is made by a methodcomprising: (a) mixing an unfractionated kaolin clay with water to forman aqueous suspension; (b) combining a coarse kaolin fraction with saidaqueous suspension, said coarse kaolin fraction having in the range ofabout 20% to about 40% by weight particles smaller than about 2 microns;(c) attrition grinding said aqueous suspension such that the averageshape factor of the kaolin clay is increased by at least about 10; (d)classifying the resultant suspension of ground kaolin clay to obtain apigment product therefrom.
 14. A method of making a pigment productcomprising a steep, platy kaolin having a particle size distributionsuch that at least about 85% by weight of the particles have anequivalent spherical diameter less than about 2 μm and from about 15% toabout 20% by weight of the particles have an equivalent sphericaldiameter less than 0.25 μm, the particles have a shape factor in therange of from about 26 to about 36, and the particles having a particlesteepness in the range of about 35 to about 40, the method comprising:(a) mixing an unfractionated secondary kaolin clay with water to form anaqueous suspension; (b) combining a coarse kaolin fraction with saidaqueous suspension; (c) attrition grinding said aqueous suspension suchthat the average shape factor of the kaolin clay is increased by atleast about 10; (d) classifying the resultant suspension of groundkaolin clay to obtain a pigment product therefrom.
 15. A methodaccording to claim 14, wherein said coarse kaolin fraction haspreviously been ground and classified to reduce its content ofagglomerated fine kaolin particles and to partly delaminate thekaolinite stacks.
 16. A method according to claim 14, wherein saidcoarse kaolin has been prepared by: (a) mixing a raw or partiallyprocessed kaolin clay with water to form an aqueous suspension; (b)attrition grinding said aqueous suspension; (d) classifying theresultant suspension of ground kaolin clay to obtain a coarse fractiontherefrom.
 17. A method according to claim 16, wherein said attritiongrinding occurs in multiple stage.
 18. A method according to claim 14,wherein in (c), the average shape factor is increased by at least about15.
 19. The method according to claim 14, wherein the aqueous suspensionis further treated with a water soluble bleaching agent.
 20. A coatingcomposition for use in producing gloss coatings on paper and othersubstrates, wherein said composition comprises an aqueous suspension ofa particulate pigment and an adhesive, wherein the particulate pigmentcomprises the pigment product according to claim
 1. 21. A compositionaccording to claim 20, wherein the solids content of the composition isgreater than about 60% by weight of the composition.
 22. A coatingcomposition for use in producing gloss coatings on paper and othersubstrates, wherein said composition comprises an aqueous suspension ofa particulate pigment and an adhesive, wherein at least about 80% byweight of the particulate pigment comprises the pigment productaccording to claim
 1. 23. A composition according to claim 20, whereinthe adhesive comprises a modified or unmodified starch.
 24. Acomposition according to claim 20, wherein the adhesive comprises abinder other than starch.
 25. A method of forming a gloss coating onpaper, comprising coating the paper with a composition according toclaim 20 and calendering the paper to form a gloss coating thereon.